The present invention relates to signal processing and more particularly to a method and an apparatus for estimating the fading distortion to which a source signal is subjected when it is transported through a communication channel. This estimation allows compensating for the fading distortion on the data symbols at the receiver, thus improving performance.
PSK (phase shift keying) modulated signals may use non-uniformly or uniformly spaced pilot symbols that are embedded in the slot structure to permit coherent demodulation at the receiver. To permit successful demodulation, especially in fast fading channels, accurate fading information is necessary in order to compensate for the fading distortion.
A communication channel, such as a channel established between a mobile telephone and a base station experiences an undesirable fading phenomenon that is due to the relative movement of the telephone to the base station. The fading is the result of a change of phase as created by the Doppler effect. When all the spectral components of the source signal are affected in the same manner the distortion is called xe2x80x9cflat fadingxe2x80x9d.
The flat fading process, whose bandwidth is limited by the maximum Doppler frequency, is a slow process compared with the source signal bandwidth. For example, at 1.95 GHz carrier frequency, the maximum Doppler frequency is 184 Hz for a mobile moving at 100 km/hour, while the baud rate of signal is 24.3 kbaud/s for narrow band TDMA systems. Prior art receiver systems compensate for the fading process by passing the data symbols through a channel equalization module. Such a module usually has two operative stages namely training and tracking. The pilot symbols embedded in the slot structures of the signal are used to complete a training sequence. Once trained the channel equalization module can then compensate the signal carrying the data symbols to reduce the effect of the channel fading. The problem with this approach is that the tracking may become inaccurate when the channel is a fast fading one. As a result, the compensation process is not always sufficiently effective to overcome the distortions in the signal.
The present invention provides an apparatus for generating a control signal indicative of the level of distortion to which data symbols in a source signal are subjected when transported via a fading channel. The source signal includes pilot symbols at known locations. Those pilot symbols are non-uniformly spaced.
The apparatus includes a first processing functional block that receives the source signal. The first processing functional block estimates the fading distortion at the pilot symbols locations. The estimate is then passed to a second processing functional block that interpolates the results to the data symbols locations. The output of the second processing functional block is a control signal that can be used to drive a compensation module processing the received data symbols in order to reduce the effect of the fading process.
In a specific example, the second processing functional block includes an input to receive interpolation coefficients that allow the interpolation operation to be performed. These interpolation coefficients are computed offline. As a result, the computational complexity of the process is small, thus facilitating its implementation.
The invention also extends to a receiver for the source signal subjected to fading distortion, to a method for processing a source signal to compensate for fading distortion and to a communication system including a receiver that can compensate for a fading distortion on a source signal.